1. Technical Field
Example embodiments generally relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device.
2. Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such image forming apparatuses are requested to print quickly while saving energy. To address this request, the fixing device that consumes energy may employ an endless belt having a decreased heat capacity that facilitates quick heating of the endless belt so as to shorten a warm-up time taken for the fixing device to be heated to a desired fixing temperature to fix the toner image on the recording medium and a first print time taken to output the recording medium bearing the fixed toner image onto an outside of the image forming apparatus after the image forming apparatus receives a print job.
Additionally, since the image forming apparatuses are requested to print quickly, an increased number of recording media is conveyed through the fixing device per minute. Accordingly, the fixing device requires an increased amount of heat to be supplied to the recording media. Consequently, upon start of a print job for printing on a plurality of recording media continuously, the fixing device may suffer from shortage of heat.
To address those requests, a fixing device 20R1 employing an endless belt is proposed as shown in FIG. 1. As shown in FIG. 1, the fixing device 20R1 includes an endless belt 21R rotatable counterclockwise in FIG. 1; a tubular, metal thermal conductor 22R stationarily disposed inside the endless belt 21R to guide the endless belt 21R; a heater 23R situated inside the metal thermal conductor 22R to heat the endless belt 21R through the metal thermal conductor 22R; and a pressure roller 24R pressed against the metal thermal conductor 22R via the endless belt 21R to form a fixing nip N between the endless belt 21R and the pressure roller 24R. As the pressure roller 24R rotates clockwise in FIG. 1, the endless belt 21R rotates counterclockwise in FIG. 1 in accordance with rotation of the pressure roller 24R. The metal thermal conductor 22R heated by the heater 23R in turn heats the endless belt 21R entirely, shortening the first print time and overcoming shortage of heat.
In order to shorten the first print time further while saving energy, a fixing device 20R2 without the metal thermal conductor 22R is proposed as shown in FIG. 2. The fixing device 20R2 includes a nip formation plate 126R disposed inside the endless belt 21R instead of the metal thermal conductor 22R depicted in FIG. 1. Since the nip formation plate 126R supported by a support 127R is disposed opposite the pressure roller 24R via the endless belt 21R at the fixing nip N, the heater 23R heats the endless belt 21R directly in a circumferential span of the endless belt 21R where the nip formation plate 126R is not interposed between the heater 23R and the endless belt 21R, thus heating the endless belt 21R effectively and therefore shortening the first print time while saving energy.
The heater 23R spans a width of a maximum recording medium in an axial direction of the endless belt 21R. Accordingly, after a plurality of small recording media is conveyed over the endless belt 21R continuously, a non-conveyance span situated at both lateral ends of the endless belt 21R in the axial direction thereof where the small recording media are not conveyed may overheat because the small recording media do not draw heat from both lateral ends of the endless belt 21R. Consequently, both lateral ends of the endless belt 21R and the pressure roller 24R contacting the endless belt 21R may be heated to a temperature high than their heat resistant temperature. To address this circumstance, it may be necessary to suppress heating of the non-conveyance span of the endless belt 21R to protect the endless belt 21R and the pressure roller 24R, degrading productivity of the fixing devices 20R1 and 20R2. Since the thermal capacity of the endless belt 21R is decreased to shorten the warm-up time while saving energy, the endless belt 21R is susceptible to overheating.